#!/bin/bash
# Rebuild a cached docker image if the input files have changed.
# Usage: ./rebuild-image <image name> <image dir> <image files...>

set -eux

IMAGENAME=$1
# shellcheck disable=SC2001
SAVEDNAME=$(echo "$IMAGENAME" | sed "s/[\/\-]/\./g")
IMAGEDIR=$2
shift 2

INPUTFILES=( "$@" )
CACHEDIR=$HOME/docker/

# Rebuild the image
rebuild() {
    mkdir -p "$CACHEDIR"
    rm "$CACHEDIR/$SAVEDNAME"* || true
    docker build -t "$IMAGENAME" "$IMAGEDIR"
	docker save "$IMAGENAME:latest" | gzip - > "$CACHEDIR/$SAVEDNAME-$CIRCLE_SHA1.gz"
}

# Get the revision the cached image was build at
cached_image_rev() {
	find "$CACHEDIR" -name "$SAVEDNAME-*" -type f | sed -n 's/^[^\-]*\-\([a-z0-9]*\).gz$/\1/p'
}

# Have there been any revision between $1 and $2
has_changes() {
	local rev1=$1
	local rev2=$2
	local changes
	changes=$(git diff --oneline "$rev1..$rev2" -- "${INPUTFILES[@]}" | wc -l)
	[ "$changes" -gt 0 ]
}

commit_timestamp() {
	local rev=$1
	git show -s --format=%ct "$rev"
}

cached_revision=$(cached_image_rev)
if [ -z "$cached_revision" ]; then
	echo ">>> No cached image found; rebuilding"
	rebuild
	exit 0
fi

echo ">>> Found cached image rev $cached_revision"
if has_changes "$cached_revision" "$CIRCLE_SHA1" ; then
	echo ">>> Found changes, rebuilding"
	rebuild
	exit 0
fi

IMAGE_TIMEOUT="$(( 3 * 24 * 60 * 60 ))"
if [ "$(commit_timestamp "$cached_revision")" -lt "${IMAGE_TIMEOUT}" ]; then
	echo ">>> Image is more the 24hrs old; rebuilding"
	rebuild
	exit 0
fi

# we didn't rebuild; import cached version
echo ">>> No changes found, importing cached image"
zcat "$CACHEDIR/$SAVEDNAME-$cached_revision.gz" | docker load
